fix(idempotency): bind operations to their own key prefix on a shared store

[svozza]

Summary

BasePersistenceLayer keeps the key prefix as mutable state and every IdempotencyHandler construction reconfigures it, so an operation that awaited a nested idempotent call completed under the nested operation's prefix and overwrote its record. Operations now capture the prefix the store resolved for them and re-apply it immediately before each persistence call. configure() also resets the prefix to its base when neither keyPrefix nor functionName is given, so an operation without a prefix no longer inherits a sibling's.

Changes

• Reset idempotencyKeyPrefix to the base value in configure() when no prefix or function name is supplied
• Snapshot the resolved prefix in IdempotencyHandler and restore it on the store before save, get, update, and delete calls
• Remove the now unused #keyPrefix field
• Add tests for nested operations with and without an explicit prefix, for the configure() reset, and one pinning that the key is hashed before the first await

Issue number: closes #5707

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